Update upgrade_notes to mention fail fast on workflows

[amanda11]

Included a comment on upgrade notes about fail-fast on orquesta workflows.

Designed to explain the confusion on when workflows stop, raised in StackStorm/st2#5031.

[nmaludy]

@emptywee any thoughts here?

[emptywee]

Yes, sounds good. Would be better if we had a choice in how we want the engine to behave (like I proposed to have fail and terminate and one would keep other branches running, while the other would terminate the workflow for good), but for now it is more clear to understand why your ex-mistral workflows converted to orquesta don't work the same way.

[m4dcoder]

I don't know why this specific feature deserves a mention in the upgrade notes. There are other Mistral -> Orquesta nuance that users should be aware as well. Why not just ask users to review the orquesta documentation in general?